Physical and Numerical Modelling of Pipelines in Elastic Visco-Plastic Soils Under Long-Term Ground Movements
Date
2018-05-15
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Abstract
Pipelines have been constructed and designed to transport essential natural resources such as water, oil, and natural gas for the last 160 years. Many pipeline sections built are buried at shallow depths below the ground surface and through different geologic terrains. This includes complex terrains where permanent ground deformations can occur such as moving slopes/landslides, surface faulting, and ground subsidence. Pipeline sections subjected to such permanent ground deformations can yield because of large strain accumulation over time leading to large pipe stresses. The mode of pipe yielding promoted by permanent ground deformations is dependent on the pipeline axis orientation with respect to the ground movement direction. Longitudinal ground movements promote pipe buckling, transverse ground movements promote pipe bending, and complex ground movements promote a mix of pipe bending and buckling. This study introduces a set of numerical tools aimed at predicting the soil movement before a buried pipeline section reaches the onset of yielding. Various numerical tools are developed for pipelines buried in compacted clay soils such as Regina clay. Compacted clay soils are of specific interest as they exhibit significant time-dependent behavior such as creep, constant strain rate effect, and stress relaxation. The main benefit of the numerical tools developed in this thesis is for the pipeline industry to be able to continually assess pipeline performance in areas with unstable soil movements and to perform necessary remediation procedures such as pipe stress relief to prolong pipeline operation at the right time. The three phases of numerical tool development include: 1) Determination of constitutive models for compacted Regina clay and soil-pipe interface, 2) Validation of the longitudinal, transverse and vertical uplift soil resistances in existing buried pipeline guidelines through a series of physical soil-pipe prototype tests, and 3) Validation of the physical prototype test results through the creation of an Extended Finite Element Method (XFEM) numerical model. The physical prototype test results illustrate consistency with existing guidelines for transverse horizontal soil resistances, but a discrepancy for vertical uplift soil resistances. There also exists behavior discrepancy in the longitudinal soil resistances. The discrepancy in vertical pipe uplift results arises from neglecting the tensile failure mode of the soil in the guidelines. The behavior discrepancy in longitudinal loading is due to differences in adhesion factor estimation, also neglected in the guidelines. The final XFEM numerical model with cap (modified Drucker-Prager) plasticity coupled with Singh-Mitchell creep law for Regina clay demonstrate good predictions of soil-pipe interactions from physical prototype tests.
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Keywords
Pipelines, Clay Soil, Pipe-Soil Interaction, Elastic Visco-plastic Soil
Citation
Wong, C. K. (2018). Physical and Numerical Modelling of Pipelines in Elastic Visco-Plastic Soils Under Long-Term Ground Movements (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca. doi:10.11575/PRISM/31925